To Rick Roll someone is to upload a video on YouTube so they think it's a special video. But when they actually start the video it plays Rick Atsley's Never Gonna Give You Up.
How To Do ItEdit
Think of a common thing that is sweeping the YouTube nation (The Dark Knight, Transformers Revenge of the Fallen, Apple, etc). Upload the song but use a different name and image. E.G if you are using Prime vs Megatron in Revenge of the Fallen, use the title ROTF: Prime vs Megatron. Then, RICK ROLL they hear that old song play. The video on the bottom is a Rick Roll. The top is a howto video of how to do it.